The primer bulb on your Weed Eater-brand string trimmer -- or on any gasoline-powered small-engine power tool, for that matter -- is attached to the fuel return line, which runs from the carburettor back to the fuel tank. The return line allows excess fuel to expunge back to the tank. The primer bulb creates a similar suction effect as the pulsing crankcase, drawing fuel from the tank and into the carburettor to enable the small engine to start. It is a sort of choke mechanism that helps in cold starts.
Dirty Fuel Lines
Your Weed Eater uses two fuel lines in the fuel system: One draws fuel up from the gas tank and the other brings leftover fuel back to the tank. Both of these fuel lines need to be clean and free of obstruction to move gas through the complete fuel cycle. Due to fuel impurities, old or bad gas, and dirt or other debris inside the tank, these lines can get clogged. When the fuel lines clog, the primer bulb won't create the proper suction and will collapse because there is no pressure in the fuel system. Replace fuel lines on a yearly basis or more frequently if you operate the Weed Eater in dirty conditions, or as a troubleshooting measure when your string trimmer's gas engine appears to be operating unsmoothly.
Holes in the Fuel Lines
If you haven't replaced your Weed Eater's fuel lines in a few seasons, the rubber material will start to dry up. When the fuel lines dry out, they will start cracking. These small, almost unnoticeable holes can bring too much air into the fuel system, which will cause the fuel system to lose adequate pressure and suction. Fuel lines can deteriorate even more quickly if you've left fuel in the tank for more than 30 days. After this time the fuel will start evaporating, causing the rubber to shrink and eventually crack. Replace the fuel lines every season, and follow the manufacturer's storage guidelines.
Loose Fuel Line Connections
If you recently replaced your fuel hoses or if they are a few seasons old, the points they connect to on the carburettor may come loose. If an airtight seal isn't present on these connection points, too much air will enter the engine's fuel delivery system, causing a loss in pressure and your bulb to collapse. If you suspect sealing problems, apply a press fluid to the hoses' ends and reattach them to the carburettor. This fluid will ensure an airtight seal. You can also use a rubber grommet on the main fuel line, on the underside of the tank, to prevent any leaks from building up in the fuel tank's side.
Problems Within the Carburetor
More serious problems within the carburettor can cause sealing and loss of suction and pressure problems. Diaphragms, gaskets and jets can get dirty and crack as well. Most carburettor problems, however, will need a service mechanic due to the level of skill required in the repairs.
Faulty Primer Bulb
The rubber on the primer bulb, just as in the fuel lines, will get old, dry out and crack over time. When the primer bulb loses that airtight seal with the fuel system, the bulb will collapse. Primer bulbs need replacing as often as the fuel lines, so replacing the primer bulb and fuel lines every one to two seasons should be sufficient to keep your Weed Eater working properly.